1. Field of the Invention
This invention relates to an apparatus, and a process, for forming deposited films. More particularly, it relates to a deposited film forming apparatus and a deposited film forming process which are preferably used for producing amorphous silicon type electrophotographic photosensitive members by plasma enhanced CVD.
2. Related Background Art
In the field of image formation, photoconductive materials capable of forming light-receiving layers in light-receiving members such as photosensitive members are required to have properties such that they are highly sensitive, have a high SN ratio [light current (Ip)/dark current (Id)], have absorption spectra suited to spectral characteristics of electromagnetic waves to be radiated, have a high response to light, have the desired dark resistance and are harmless to human bodies when used. In particular, in the case of electrophotographic light-receiving members set in electrophotographic apparatus use as office machines in offices, the above harmlessness in their used is an important point.
Electrophotographic light-receiving members are available which make use of amorphous silicon (a--Si) as materials that can meet such a demand. For example, Japanese Patent Application Laid-open No. 54-86341 discloses a technique relating to an electrophotographic light-receiving member having superior moisture resistance, durability and electrical properties which makes use of a--Si in a photoconductive layer. Such a technique has brought into practical use electrophotographic light-receiving members constituted of a--Si which have been improved in electrical, optical and photoconductive properties, service environmental properties and durability and also can be improved in image quality level.
Meanwhile, the production of a--Si electrophotographic light-receiving members requires a high-level technique. Especially in the case of the electrophotographic light-receiving members, a larger area and a larger layer thickness are required, compared with other devices, and hence, it is an important factor how uniformity is ensured over the whole area and how abnormal growth of films be prevented which may occur around foreign matter during the deposition of a--Si films.
From such viewpoints, a variety of proposals have been made also on how industrially stably high-quality a--Si electrophotographic light-receiving members be manufactured. In particular, in the electrophotographic light-receiving members, what is greatly questioned is the occurrence of spherical protuberances that may cause what is called "white dots", which are fine white dots appearing on copied images. The cause of such spherical protuberances is known to be dust and also fragments caused when any films having adhered to the interior (e.g., inner walls, and surfaces of structural component parts) of a deposited film forming apparatus come off, which may adhere to a substrate or a deposited film being formed on the substrate and may serve as nuclei around which the deposited film on the substrate causes abnormal growth. As a technique for preventing such spherical protuberances from occurring, various improvements are attempted on appratus. For example, it is known that, as a material for improving adhesive properties of films adhering to inner walls and surfaces of structural component parts of a reactor and for preventing such films from coming off, a ceramic material whose surface roughness has been controlled is used in the interior of a reactor so that the films can be made to come off much less frequently from such portions. As an example of such a technique, Japanese Patent Application Laid-open No. 4-247877 discloses a technique in which the vicinity of a gas release hole is formed of a ceramic so that films can be prevented from coming off to restrain the spherical protuberances from occurring.
However, in recent years, electrophotographic apparatus are sought to be made higher in image quality, higher in speed and higher in durability than ever. In addition, for the purpose of maintenance-service cost reduction, individual component parts are required to be improved in reliability so that the maintenance may be made at longer intervals, and electrophotographic light-receiving members have become durable against repeated use for a much longer time than ever in various environments without maintenance by servicemen. Under such circumstances, conventional electrophotographic light-receiving members have room for improvement.
As matters stand like this, it has become known that even spherical protuberances which are so fine that any white dots can not be recognized on images may cause various problems. Especially when the electrophotographic process is made high-speed and toners are made to have a smaller particle diameter in order to make image quality higher, melt-adhesion may come into great question, in which any unnecessary toner having remained on the surface of a photosensitive member melts and adheres thereto during electrophotographic processing such as cleaning. It has been found that this melt-adhesion is caused by the fine spherical protuberances, which act as a beginning to which toner adheres, and come to grow gradually with repetition of image formation. Even when no melt-adhesion occurs, the fine spherical protuberances may come off during the repetition of image formation over a long period of time. In this case, it has become known that the ability of charge retention of electrophotographic photosensitive members may be damaged even if the spherical protuberances are small, and hence the potential may become low to an extent about two to 10 times the diameter of each actual spherical protuberance to cause white dots appearing on images.
In the meantime, to prevent such fine spherical protuberances, the technique which employs a ceramic material having a controlled surface roughness as stated previously may have, in the present situation, problems on electrophotographic photosensitive member production processes.
The above Japanese Patent Application Laid-open No. 4-247877 discloses that ten-point average roughness (Rz) of the surface of a ceramic may be controlled within the range of from 5 .mu.m to 200 .mu.m to prevent films effectively from coming off. Thus, the use of a ceramic material in the interior of a reactor is effective for preventing films from coming off (film come-off).
Use of the ceramic having a surface roughened as stated above can bring about an effect attributable to a good adhesion to a--Si deposited films that is inherent in ceramics and attributable to the relaxation of film stress per unit area due to an enlarged surface area.
When the surface roughness of ceramic materials is controlled, a method most commonly used is blast finishing carried out by blowing a blasting medium against the surface. However, sintered ceramics, in particular, alumina ceramics, which are considered suitable especially for preventing film come-off, commonly have so high a hardness that it is actually not easy to finish the surface uniformly at a desired value of surface roughness by blast finishing. Especially in the case of a ceramic material having a cylindrical form, it is difficult, because of its form, to blow the blast medium uniformly. In actual surface roughness, non-uniform roughness may be seen at some portions.
In instances where some portions differ in surface roughness, fine film come-off may occur in some cases in the vicinity of boundaries of areas having extremely different surface roughness, and have caused fine spherical protuberances in some cases. In order to prevent the non-uniform surface roughness, the surface roughness at each point of component parts of a reactor must be measured and thereafter the surface must be again put to blast finishing, which is done repeatedly. This, however, is not preferable in view of cost management.
It has also become clear that, even in component parts whose surfaces have been roughened uniformly by careful blast finishing, the adhesion of deposited films formed thereon may become gradually poor during the use over a long period of time. This is caused by the surface coming to be scraped because of a mechanical stress applied to ceramic component parts, e.g., their rubbing against other component parts. To avoid such a problem, the component parts are obliged to be put to periodical blast finishing to reprocess the surface. In such an instance, too, the management of surface roughness is necessary and complicated operations are required as stated previously.
In addition, depending on the shape of a component part in the reactor constituted of ceramic materials, blast finishing may be difficult at some portions even though deposited films may adhere thereto. As the result, the film come-off can not be effectively prevented in some cases.
According to experiences the present inventors have had, even when the film come-off occurs inside the reactor in the course of deposited film formation because of the use of such component parts, it occurs on a very small scale. Accordingly, the spherical protuberances appearing on the electrophotographic photosensitive member are so fine that it can be a rare case that spherical protuberances that may cause white dots on copied images come to appear on the surface of the electrophotographic photosensitive member. Under existing circumstances, however, because of the much higher demand on electrophotographic apparatus than ever in recent years as stated previously, a countermeasure is desired also against such fine spherical protuberances that may not cause white dots.